Firing line for subaqueous geophysical prospecting



Jan. 27, 1959 G. M. PAVEY, JR 2,870,708

FIRING LINE FOR SUBAQUEOUS GEOPHYSICAL PROSPECTING Filed Aug. 19, 1955 5 Sheets-Sheet 2 Fiq, 3

III

VIII/II/II/III 24 I4 27L v \1 32 23 GEORGE M. PAVEY, JR.

2| I9 34 Lafi United States Patent FIRINGLINE FOR SUBAQUEOUS GEOPHYSICAL PROSPECTING George M. Pavey, Jr., Dallas, Tex., assignor to Marine Instrument Company, Dallas, Tex.

This invention relates to firing lines and explosive charges operatively connected thereto for initiating seismic signals employed with systems for underwater exploration of subsurface geological structures and more particularly to a firing line for use with a geophysical prospecting system in which the firing impulses are applied to the firing lines from a vessel in such manner that the firing current flows through a circuit in which the sea water comprises only a small fractional portion thereof.

In certain firing circuits of the type heretofore proposed it has been the usual practice to employ a sea water return from an electrode in close spaced adjacency to the explosive charge to a plate secured to the hull of the vessel or to the hull of the vessel itself, as the case may be. Such an arrangement, for example, is disclosed and claimed in the Patent No. 2,627,930 to J. P. Woods and Patent No. 2,465,696 to Le Roy C. Paslay. In such systems, however, under extreme adverse conditions of service it sometimes happens that electrical leakage to the sea occurs in the seismic wave receiving cable and a portion of the firing current may pass through the cable and thence to the vessel. When this occurs, undesired electrical disturbances of fortuitous character are picked up by the seismic wave receiving cable and recorded on the seismic recording equipment.

The apparatus of the present invention provides new' and improved means for preventing the occurrence of such unwanted electrical disturbances by employing an arrangement in which the sea water return path for the firing current is not required. A further advantage in the firing line of the instant invention resides in the provision of new and improved means for maintaining an electrical connection between the trailing end of the firing line and the electroresponsive detonating means for firing the explosive charge until the explosive charge has been towed by the firing line to a predetermined position above the subaqueous terrain as the shot is fired.

One of the objects of the present invention is the provision of new and improved means for firing an explosive shot above a seismic spread disposed within a body of water.

Another object is to provide means for firing an explosive shot within the body of water above a seismic spread in which the firing circuit includes a return path for the firing current comprising a short effective length of sea water through which the current flows in a direction substantially normal to the direction of movement of the seismic detecting cable.

Another object is the provision of a firing line having new and improved means for firing an explosive charge connected thereto and in which the current flowing through the sea water is less than the firing current applied to the detonator for firing the explosive charge.

Another object is to provide new and improved means for maintaining an electrical connection between the trailing end of a firing line and an explosive charge 2,870,708 Patented Jan. 27, 1959 ice connected thereto and for disengaging the connecting means from the firing line selectively at will as the explosive shot is fired.

A further object resides in the provision of inertial means responsive to the force of the explosion for releasing the firing ring from the electrical contact element as the explosive charge is fired.

A still further object of the present invention is the provision of hydrostatic means for releasing the firing ring from the electrical contact element in response to a pressure wave within the water caused by the explosion of the charge.

Still other objects, advantages and improvements will be apparent from the following description taken in connection with the accompanying drawings of which:

Fig. l is a view in elevation of a seismic surveying system employing the device of the present invention in accordance with a preferred embodiment thereof;

Fig. 2 is a plan view of the system of Fig. 1;

Fig. 3 is a detail view in elevation and partially in section of an enlarged firing line having an explosive charge connected thereto in accordance with another embodiment of the invention;

Fig. 4 is a diagrammatic view of the firing line in accordance with an alternative form thereof;

Figs. 5 and 6 are diagrammatic views of the firing line in accordance with two other alternative forms thereof;

Fig. 7 is an enlarged view partially in section and partially broken away of an electrode suitable for use with the firing line of Figs. 1-6 on which is shown electrically actuatable means for releasing the firing ring therefrom when the charge has been fired;

Fig. 8 is a view taken along the line 8-8 of Fig. 7;

Fig. 9 is a view showing in diagrammatic form an electrical circuit suitable for use with the firing line of Fig. 7;

Fig. 10 is a view showing an alternative circuit arrangement for operating the ring releasing apparatus of Fig. 7;

Fig. 11 is a view showing means for operating the ring releasing apparatus of Fig. 7 selectively at will as the charge is fired;

Fig. 12 is an enlarged view partially in section on which is shown inertia actuated means for releasing the firing ring from the electrical contact element at the end of the firing line; and

Fig. 13 is a fragmentary view partially in section on which is shown hydrostatically actuated means for operating the ring releasing mechanism of Fig. 11.

Referring now to the drawings for a more complete understanding of the invention and more particularly to Fig. 1 thereof, there is shown thereon in diagrammatic form a system for making geological surveys of subaqueous terrain in which a seismic spread indicated generally by the numeral 10 is towed by a vessel 11. A spread well suited for use with the present invention is disclosed and claimed in Patent No. 2,465,696 to Le Roy C. Paslay, issued March 29, 1949, although, if desired, other spreads may be employed. Secured to the vessel and towed thereby in any suitable manner as by the pair of booms 12 are a pair of elongated tubular firing lines 13, each of said lines being insulated throughout the length thereof from the sea water by a jacket and provided with an exposed electrode or contact element 14 near the trailing end thereof. The end of the firing line is sealed by a plug or closure 15 composed of insulating material suitable for the purpose and having an electrical conductive firing rod or exposed flexible wire 16 trailing therefrom. The inner end of the conductive element 16 is connected as by conductor 17 to a source of electrical energy on the vessel, from whence the circuit is continued by way the invention herein disclosed.

of conductor 18 to the electrode contact 14, Fig. 3, the circuit including the usual firing switch whereby firing potential is applied at will between the contact 14 and the elongated electrode element 16. The interior of the firing line or cable 13 is preferably .filled with .air or is otherwise made positively buoyant whereby the firing cable floats at or near the surface of the water although, if desired, the firing line may be neutrally or negatively buoyant without departing from the spirit and scope of The explosive charge is contained within a container or casing 19 and carries an electroresponsive detonator or blasting cap 21 for firing the explosive charge. The casing 19 is preferably composed of electrical conducting material although, if desired, pastebcard or the like may be employed. In the event the casing is composed of electrical non-conducting material, a metallic wrapping, coating or band preferably is applied exteriorly thereto. When a casing .of insulating material is employed, the blasting cap wire 22 may be connected to the metallic element encircling or otherwise carried by the casing or the cap wire 22 may comprise a bared length thereof in contact with the sea water after the casing has been launched; With this latter arrangement the electroconducting element carried exteriorly by the casing would not berequired and may be dispensed with. For the purpose of description it will be assumed herein thatthe casing 19 is composed of electroconducting material and that the cap wire 22 is connected thereto. The blasting cap is connected by conductor .or blasting cap wire 22 to the conductive casing 19 and by conductor 23 to a contact element such as the metallic ring 24 illustrated of a diameter sufficient to slide easily along the firing cable and along the elongated contact surface of the electrode contact 14 until further movement thereof along the contact surface is arrested as by an annular protuberance 25' formed thereon. I

' applied to electrode contact 14 and firing electrode 16,

the firing circuit being completed from contact 14 by way of ring 24, conductor 23 to the detonator 21 from whence the circuit is continued by way of conductor 22, container 19 and through the sea water to the conductive firing electrode 16, the voltage between the firing rod 16 and container 19 being suflicient to operate the detonator. An arrangement is thus employed in which a long sea water return path from the explosive shot to the vessel is not employed and thus there is no possibility of fortuitous and erroneous seismic signals being applied to the seismic recording apparatus by reason of the leakage of such firing current into the seismic detectin streamer.

In operation, it has been found advantageous to employ one of the firing lines only for firing the explosive charges in successive order until :a predetermined .number of firing rings have accumulated on the electrode contact element 14. When this occurs, the firing opera tions are transferred to the other firing line and,the.first firing line is retrieved for removal of the :firing ringstherefrom.

Whereas Figs. 1 and 2 have been described with reference to a trailing firing rod or wire electrode '16 protruding therefrom, and in which each .firing operation employs a sea water return between the casing 19 and the rod 16, the invention is not so limited as .a-firing circuit may be established by applying the firing potential to the electrodes 14 of both firing lines simultaneously such that a firing potential .is established between the electrodes 14, in lieu of applying firing potential between the electrode 14 of one firing line only and the trailing electrode 16 associated therewith. When this two line arran'gement is employed the firing circuit may be traced from electrode 14 of one of the firing lines, ring.24 carried thereby, conductor '23 to the 'blastingacap 21 from whence the circuit 'is-continued by way of the conductor carried thereby for completion of .the firingcircuit by way 2'2, casing 19 and sea water to the electrode 14 of the other firing line, the path of the current through the sea water being substantially normal to the length of the detector streamer and transversely thereof. Explosive charges may be connected to either of the firing lines and fired at will when this arrangement is employed.

It has been found advantageous to assemble a num ber of rings 24 on the firing lines and make the conductors 23 secured thereto fast at the vessel. When a charge is to be fired, it is secured to the conductor 23 on the vessel, the conductor 23 is released and the conductor and charge are lowered into the water. As the vessel continues forward movement the firing line slides through ring 24 until it is arrested by the protuberance 25 on the contact element 14. When this occurs the charge is towed by the vessel until the firing switch is actuated to explode the charge.

On Fig. 4 is shown an arrangement in which a transformer 26 is provided within the trailing end portion of the firing line. former is connected by conductor 28 to the trailing electrode 16 and by conductor29 to a firing switch 31 whereby closure of the firing switch applies current from a di rect current source .32 to the electrode 16, the circuit being completed through the sea water to a plate 33 secured to the vessel exteriorly below the water line and electrically connected to the current source 32 as by conductor 34. The transformer 26 is also provided with a secondary winding 35 connected to contact element 14 whereby the suddenly induced voltage in winding 35 as switch 31 is closed is applied to conductor 23 to fire the charge, the firing circuit including a short sea water return portion between the container 19 and the trailing electrode 16 through which the .firing current flows in a direction transverse to the longitudinal direction of the seismic detection streamer. The transformer 26 may be regarded as a voltage step-down or current step-up transformer whereby the current appliedto the blasting cap within the container 19 isgreatly-in excess of the current flowing through the primary winding 27 and through the sea water between the tail rod 16 and the plate 33. This primary current is sufiiciently small as to prevent signals being transmitted to the seismic recording apparatus as the result of leakage of the seismic detection streamer. This beneficial result is achieved-by employing the transformer arrangement of Fig. 4 having a high impedance primary winding 27 and,

a source of potential 32 sufiiciently high to generate a firing current in winding 35 in response to closure of th firing switch 31.

On Fig. 5 is shown a firing arrangement, similar to the arrangement of Fig. 4, in which the circuit to the primary winding 27 of transformer 26 includes two conductors within the firing cable 13 and the sea water return between the electrode 16 and the plate 33 carried by the vessel is not employed.

There is shown on Fig. 6 a modification of the firing arrangementof Fig. 4 .inwhich the primarywinding 27.of the transformer 26 is connected to an auxiliary grounding electrode such,,for example, as the annular element 30.

illustrated. This electrode 30 is carried exteriorly by the firing line 13 in close spaced adjacency but not in contact with electrode 14. With this arrangement the primary circuit for transformer 26 includes a sea water return between the electrode 30 and the plate 33 on the vessel and excludes the firing electrode 16, which carries only the secondary current induced in the transformer winding 35 for firing the explosive charge as the firing switch v31 is closed.

On Fig. 7 is shown an electrode 36 suitable for use with the firing lines .13 and comprising a metallic contact element 37 of tubular configuration and composed ofmetal suitable for the purpose such, for example, as brass adapted for connection to the firing line 13. The end of the contact element '37 is enclosed :by a plug or closure 15 having a .tailrod, wire or other firing electrode 16 The primarywinding 27 of the trans- 1 of sea water therefrom. to the container 19 of the explosive charge. Supported within the contact element 36 as by screws 40 and carried thereby is an electromagnet 38 having a core 39 threaded to receive the screws 40 and adapted to be energized by winding of coil 41, a pair of terminals 42 being provided for establishing an external electrical connection to the winding. The core 39 is provided with a heel portion slotted or otherwise formed at- 43 to receive a pair of magnetic members or arms 44 pivoted thereto as at 45. Each of the arms is provided with an L-shaped extension or stop 46 disposed within apertures 47 formed within the contact element 37 when the arms are movably urged outwardly by spring 48 connected thereto to an initial stop position, as shown on Fig. 7, such that ring 24 is brought to rest thereagainst before the shot is fired. Upon energizatic-n of winding 41, arms 44 are quickly moved against core 39 in which position the ends of the stop members 46 are disposed within the outer configuration of contact element 37 releasing ring 24 which thereupon is disengaged from contact element 36 and is expended.

A circuit suitable for use with the device of Fig. 7 is shown on Fig. 9. In this circuit arrangement closure of firing switch 31 applies firing potential to contact element 37 and trailing electrode 16. Coil 41 is energized by this potential withdrawing stop members 46 to release ring 24. In the arrangement of Fig. 9, coil 41 is connected in parallel with the blasting cap and both the ring release magnet 38 and the blasting cap, by reason of this parallel arrangement, operate substantially at the potential applied to the firing line 13 by closure of firing switch 31.

On Fig. is shown a circuit arrangement for actuating the ring release magnet 38 in which the coil 41 thereof is energized in series with the blasting cap in response to a firing impulse applied to the firing line 13. In the circuit arrangement of Fig. 10. a blasting cap possessed of slow acting characteristics is desired in order that the cur rent would be applied to the coil 41 for a period of time sufficient to operate electromagnet 38 and release the ring 24 before the circuit is interrupted by the explosion of the cap. The delay in the firing of the blasting cap is of such value as to insure that ring 24 is released and that the cap will invariably fire before the ring has broken contact with the contact element 37.

On Fig. 11 is shown a firing arrangement generally similar to the arrangement of Fig. 9. On Fig. 11, however, the firing switch 31 is replaced by a firing switch 49 preferably of the double pole double throw type such that a firing potential of either polarity may be applied selectively to the firing line 13. The coil 41 of electromagnet 38 is connected to a rectifying element 51 in series therewith whereby the magnet 38 is operated selectively in accordance with the polarity of the firing potential applied to the firing line. If it is desired, for example, to fire the explosive charge and not release ring 24, switch 49 is operated to the Fire position. If, however, a ring 24 is to be released as the charge is fired, switch 49 is thrown to the Fire and Release position whereby the firing current flows through the firing line in a direction to energize winding 41 and operate the ring release magnet 38.

On Fig. 12 is shown an alternative form of ring release mechanism in which the release of the contact ring 24 is achieved through the operation of an inertial element in response to the sudden shock received by the tail portion of the firing line as the explosive shot is fired. This arrangement comprises a contact element 52 fitted with a closure at one end thereof and secured thereto. The other end portion fits into the trailing end of the firing line 13. The contact element 52 is generally similar in outward appearance to contact element 37 and may, if desired, be composed of the same material as contact element 37. Contact element 52 is provided with a plate 53 secured transversely and interiorly thereto and having an aperture 54 centrally arranged therein.

' The inertial element comprises a mass 55 carried by a support 56 secured to or formed integrally with a circular base member 57 having a flat surface engageable by plate 53. There is secured to the support 56 in any suitable manner a rod 58 of less diameter than aperture 54 within which the rod is disposed and connected as by the swivel sockets 59 to a link 61 and an actuating member 62 substantially as shown. The actuating member 62 has connected thereto a spring 63, one end of which is secured as by a pin 64 to the member 52 whereby the spring 63 exerts sufficient tension on the rod 58 to maintain the inertial member against plate 53 as shown until the device receives a violent shock or impact applied laterally thereto of suflicient force to cause pivotal movement of the mass 55 about the outer periphery of base member 57. Such pivotal or rocking movement causes actuating member 62 to be moved to the right as viewed in Fig. 12 of the drawings and concurrently therewith elongating and increasing the tension in spring 63.

Member 62 is provided with an elongated slot 65 to which is connected as by the pins 66 a pair of ring releasing members 67. Members 67 are pivotally connected to the contact element 52 as by the pins 68 and supporting members 69 illustrated. The outer ends of the ring releasing members 67 extend through apertures 71 formed in the contact element 52 sufficiently to lock ring 24 thereto until the outer ends are retracted in response to movement of the inertial mass 55. The device of Fig. 12, it will be understood, is constructed and arranged in such manner that pivotal movement of mass 55 in response to the explosion of the explosive charge as a sudden violet impulse or shock is received therefrom causes actuation of ring releasing members 67 and release of ring 24 regardless of the instant oriented position of the device with respect to the pressure front of the explosive wave produced by the firing of the seismic shot.

Whereas the device of Fig. 12 has been described and illustrated with reference to two ring releasing members 67 it will be understood that this is by way of example only and that the number of releasing members may be varied without departing from the spirit and scope of the invention herein disclosed. V I

I-Iydrostatically responsive means for actuating the ring releasing mechanism of Fig. 12 is shown on Fig. 13, the device comprising a contact element 60 generally similar to contact element 52 and having the ring releasing members 67 and associated control elements carried thereby. The contact element 60 diifers from contact element 52, however, in the provision of :a plurality of apertures 70 formed therein and a removable plate 72 to which is secured in any suitable manner a compressible bellows 73, the plate 72. and apertures 70 forming a pressure sensitive chamber within which the bellows is disposed. The interior of the bellows is in fluid communication with an air chamber formed between plate 72 and closure 15 substantially as shown. The movable end of the bellows is pivotally secured to the link 61, Figs. 11 and 12, as by the member 74. With this arrangement the pressure wave generated by the explosion of the charge causes a sudden increase of hydrostatic pressure within the pressure sensitive chamber and received throu h apertures 70 to be applied exterio-rly to bellows 73. When this occurs, the bellows is contracted thereby moving link 61 to the right as viewed in the drawings and releasing ring 24 from contact element 60. Whereas the pressure responsive means disclosed in Fig. 13 comprises a compressible bellows, it will be understood this is by way of example only as other arrangements and pressure responsive instrumentalities suitable for the purpose such, for example, as an expansible bellows, flexible diaphragm, piston or the like may be employed.

Furthermore, while the invention has been disclosed with reference to a pair of booms for paying out and retrieving at will the firing lines from opposite sides of the vessel it will be understood that, if desired, the booms may be omitted and the firing lines may be payed out 7 directly from opposite sides of the vessel without employing a boom arrangement.

In rough weather or with heavy cross winds it has been found difficult to employ a pair of firing lines .for' the reason that there is a possibility that the upwind line may assume a position within the water such that the, explosive charge may become entangled with the cable dragging the seismic detection streamer. Under such weather conditions, seismic operations may be continued uninterruptedly for an indefinite period by employing only one firing line on the downwind 'side of the streamer and jettiso-ning the firing rings as each shot is fired by employing the ring releasing'mechanism disclosed herein.

Furthermore, if desired, the ring releasing mechanism may be sealed against the entrance of-water by providing waterproof seals composed of flexible rubber or any 'of the'synthetic varieties thereof about stop members 46 or 67 and sealing the peripheral portion of such seals to the contact element 37 or 52, as the case may be, and by providing a flexible seal about member 74. When this arrangement is employed the water baflle member and electrical gland carried thereby for excluding water from the interior of the firing line 13 would not be required and may be omitted.

Whereas the ring 24 has been shown on the drawings as of continuous one piece construction it is to be understood that the invention is not so limited as the ring, if desired, may comprise two arcuate portions, one of'which is hinged or otherwise movable with respect to the other, and means such as a spring or the like for maintaining the ring closed after it has been placed on the firing line. When the booms 12 are not employed the rings may be placed on the firing line in successive order as required and jettisoned as each shot is'fired whereby an unlimited number of shots may be fired from one line without interruption of the seismic operations. Furthermore, the container 19 may be directly connected to the firing ring and insulated therefrom and one of the leads to the blasting cap21 would be connected to the ring 24. With this arrangement conductor 23 would not be required to supply an electrical and towing connection to the cap and container respectively.

While the invention has been described with particular reference to several examples thereof which give satisfactory results, it will be understood by those skilled in the art to which the invention pertains, after understanding the invention, that various changes and modifications may be made without departing from the spirit and scope of the invention and it is my intention, therefore, in the appended claims to cover all such changes and modifications.

What I claim as new and desire to be secured by Letters Patent of the United States is:

1. In a surveying system for seismic exploration of submerged areas from a vessel, the combination of a firing line towed by said vessel and having an electrode exposed to the surrounding seawaternear the trailing end thereof, said electrode comprising a contact surface encircling the firing line and having a stop member formed thereon, a casing having an explosive charge therein, said casing comprising means forming a second electrode, an electroresponsive detonator for exploding said charge, means for electrically connecting the detonator to said first named and second electrodes respectively, a third electrode on the trailing end of said firing line, said third electrode being constructed and arranged to be in close spaced adjacency but not in physical contact with said second electrode and to extend rearwardly beyond said trailing end of the firing line as the firing line is towed through the water, and means for applying an electrical firing impulse to said first named and third electrodes of sufiicient strength to fire the detonator.

2. The combination according to claim 1 in which the means for connecting the detonator to the first named electrode comprises a contact member slidable on'said contact surface and adapted to be stopped by said stop member while the firing line is being towed through the water.

3. The combination according to claim 2 in whic said third electrode comprises a firing rod insulated from said first electrode and secured to the firing line in trailing relation therewith.

4. The combination according to claim 2, in which said third electrode comprises a length of bared flexible wire insulated from said first electrode and secured to the firing line in trailing relation therewith.

5. In a system for seismic prospecting of submerged areas from a vessel, in combination, a pair of firing lines towed by said vessel in mutually spaced adjacency, each of said lines having an exposed electrode near the trailing 'end portion thereof in contact with the sea; water, a

container having an explosive charge therein and operatively connected to one of said firing lines near the trailing end portion thereof in such manner that the container is substantially opposite the respective electrode when the lines are towed through the water, said container comprising means forming an electrode element in contact with the sea water, an electroresponsive det onator for firing said charge and having a pair of electrical conductors connected to one of said electrodes and to said electrode element respectively, and means for applying a firing voltage differential to said electrodes suificient to actuate said detonator and fire the charge.

6. The combination according to claim 5 in which said firing lines are of equal length whereby the firing circuit g for said detonator includes an aqueous portion disposed between said electrode element and the other one of said electrodes through which the firing current flows in a direction transverse to the direction of movement of said firing lines.

7. In a system for subaqueous geophysical prospecting from a vessel having a source of current thereon, in

combination, a normally open firing switch connected. to said current source, a firing line having a conductor connected to said switch and comprising a flexible elongated jacket adapted to be towed through the sea water by the vessel, a closure at the trailing end of said jacket, an elongated firing electrode secured to the trailing end of said closure and in contact with the sea water, a trans former sealed within said jacket at the trailing portion 1 thereof, means disposed within said jacket for connecting the primary winding of said transformer to said firing electrode and to said conductor respectively, a second electrode operatively connected to said source of current and in contact with the sea water whereby said primary winding is energized as the switch is closed, a secondary winding on said transformer having one end thereof connected to said firing electrode, a contact element carried exteriorly by said jacket at the trailing portion there of in contact with the sea water and electrically connected to the other end of said secondary winding, 3 coni tact member constructed and arranged to slide easily along said jacket and said contact element, means on said contact element for arresting further movement of r 1 said contact member as the contact member is engaged 1 current induced in said secondary winding by sudden energization of the primary winding as the firing switch is closed, the induced firing current applied to said eltk' troresponsive detonator also flowing through the sea water intermediate said third electrode and the firing electrode.

8. The combination according to claim 7 in which the-impedance of the primary winding of the transformer exceeds the impedance of the secondary winding whereby the value of the current flowing through said current source is less than the current flowing through the det onator.

9. In a system for subaqueo-us geophysical prospecting from a vessel having a source of current thereon, in combination, a normally open firing switch connected to said current source, a firing line having a conductor connected to said switch and comprising a flexible elongated jacket adapted to be towed through the water by the vessel, a closure at the trailing end of said jacket, an elongated firing electrode secured to the trailing end of said closure and in contact with the sea water, a transformer sealed within said jacket at the trailing portion thereof, a second electrode carried by said line at said trailing portion, means disposed within said jacket for connecting the primary winding of said transformer to said conductor and to said second electrode respectively, a third electrode operatively connected to said source of current and in contact with the sea water whereby said primary winding is suddenly energized as the switch is closed, a secondary winding on said transformer having one end thereof connected to said firing electrode, a contact element carried exteriorly by said jacket at the trailing portion thereof in contact with the sea water and electrically connected to the other end of said secondary Winding, a contact member constructed and arranged to slide easily along said jacket and said contact element, means on said contact element for arresting further movement of said contact member as the contact member is engaged thereby, a casing having an explosive charge therein, a fourth electrode carried by said casing, an electroresponsive detonator disposed within said casing for firing said charge, means electrically connecting said electroresponsive detonator to said contact member and to said fourth electrode whereby the explosive charge is fired by current induced in said secondary winding by sudden energization of the primary winding as the firing switch is closed, the induced firing current applied to said detonator also flowing through the sea water intermediate said fourth electrode and the firing electrode.

10. In a seismic system for underwater surveying, in combination, a vessel having a source of electrical potential thereon, a normally open firing switch connected to said potential source, a firing line adapted to be towed by the vessel within the sea water and having a transformer carried thereby, a pair of conductors arranged within said firing line and connected to said switch, to the primary winding of said transformer and to said potential source in series whereby the transformer is suddenly energized as the switch is closed, a firing electrode trailing from said firing line in contact with the sea water, means for connecting one terminal of the secondary winding of said transformer to said firing electrode, a contact element encircling the trailing end portion of said firing line and in electrical connection with the other terminal of said secondary winding, a contact member slidable on said firing line and engageable by said contact element, a casing having an explosive charge therein, said casing having electrode means for establishing an electrical contact with the surrounding water, an electroresponsive detonator having two electrical leads and carried by said casing for firing the charge, means for electrically connecting one of said leads to said contact member and the other lead to said electrode means, means for maintaining the contact member in engagement with said contact element while the firing line is being towed by the vessel, and a firing circuit for said detonator including said contact element, said leads, said contact member and an aqueous portion between said electrode means and said firing electrode whereby the detonator is energizable by current induced in said secondary winding as the switch is closed.

11. In a system for seismic prospecting of submerged areas from a vessel, in combination, a firing line disposed within the sea water and adapted to be towed by the vessel, a contact member encircling the trailing end portion of said line, a metallic contact element slidable on said line and contact member, a container having an explosive charge therein, an electroresponsive detonator disposed within said container for firing said charge, electrode means carried by said container, means electrically connecting said detonator to said contact element and said electrode means respectively, a firing electrode secured to and trailing from said firing line, a firing circuit com prising a pair of conductors within said firing line and connected to said contact member and said firing electrode respectively, a normally open firing switch in said circuit, an electroresponsive device having means for releasably locking said contact element on the contact member when engaged thereby, and means operatively connecting said electroresponsive device to said pair of conductors whereby the locking means is actuated by the device to release said contact member as a firing impulse is suddenly applied to said pair of conductors by closure of the firing switch.

12. The combination according to claim 11 in which current rectifying means are provided in series with said electroresponsive device for rendering the electrorespon sive device responsive to a sudden firing impulse of a predetermined polarity and unresponsive to a sudden firing impulse of the opposite polarity.

13. The combination according to claim 12 in which means are provided for applying at will a sudden firing impulse of either polarity to said pair of conductors.

14. In a system for seismic prospecting of submerged areas from a vessel, in combination, a firing line adapted to be towed through the sea water by the vessel, an explosive charge, electroresponsive means for firing said charge, a metallic contact member electrically connected to said firing means and slidable on said line for establishing a firing connection thereto, a complementary contact element on the trailing end of said firing line for establishing an electrical connection to said contact member as the element is engaged thereby, movable means on said complementary contact element for arresting movement of the contact member as the member is engaged thereby, means for applying a firing impulse to said contact element, an inertial element carried by said line at the trailing portion thereof, and means carried by said contact element and operatively connected to said inertial I element for unlocking said contact member as the inertial element is actuated in response to the shock impact of the explosion of said explosive charge.

15. The combination according to claim 14 in which said contact member unlocking means comprises a plurality of locking elements movable from an initial locking position to a release position as the inertial element is actuated by said shock impact.

16. The combination according to claim 15 in which resilient means are provided for restoring the locking elements to said initial locking position when the force of said explosion has subsided.

17. In a system for seismic prospecting of submerged areas from a vessel, in combination, a firing line adapted to be towed through the sea water by a vessel, a contact element on the trailing end of said firing line, a container having an explosive charge therein and comprising an electrode element for exposure to the surrounding sea water, electroresponsive means for firing said charge and having a pair of firing leads connected thereto, one of said leads being connected to said electrode element, a complementary electroconductive contact member electrically connected to the other of said leads and slidable on said line and said contact element for establishing an electrical connection from the firing means to the contact element as the element is engaged thereby, movable means on said contact element for arresting movement of the contact member as the member is engaged thereby, a firing electrode secured to said line in trailing relation therewith, means for applying an electrical firing impulse to said contact element and firing electrode thereby to actuate said firing means and fire the charge, means forming a-pressure sensitive chamber within the trailing end 'of said line, a hydrostatic element disposed within said chamber and movable from an initial position of rest to a moved position in response to the pressure wave Within the water caused by the explosion of said charge, and means operatively connecting the hydrostatic element to member is released from said contact element as the hydrostatic element moves to said moved position.

References Cited in the file of this patent I r said movable means whereby the complementary cont act 10 2, 7,93

UNITED STATES PATENTS Flude July 13,, 1943 Holm July 23,1946 Paslay Mar. 29, 1949 Woods Feb. 10, 1.953 

